The present invention relates to an engine-driven permanent magnet type welding generator, and more specifically to a welding generator provided with a rotating magnetic field system constructed by use of permanent magnets.
When welding work is conducted in the open air, in general a portable-type engine-driven welding machine assembled with an electric generator is widely used. This is because in the environment where a power source is difficult to obtain as with the case of the open air, it is necessary to supply a power source not only to the welding welder but also to some other apparatus (e.g., machining apparatus and illumination apparatus).
Therefore, the welding generator constructed as a synchronous generator of rotating magnetic field type is widely used together with the welding machine, to supply an auxiliary power in addition to a welding power.
In the case of the welding generator of this type, since an auxiliary power source of commercial power frequency is required according to the number of engine revolutions, the welding generator is constructed in such a way that the number of poles is two, the revolution speed thereof is 3600 rpm, and the frequency is 60 Hz, or else that the number of poles is four, the revolution speed thereof is 1800 rpm, and the frequency is 60 Hz. On the other hand, the welding power is outputted as a dc arc welding output obtained by full-wave rectifying three-phase ac output.
As the result that the rotating magnetic field system is obtained by use of two or four poles, when a welding generator 1 and an engine 2 both shown in FIG. 16 are coupled with each other, the size of the welding machine provided with the welding generator inevitably increases. For instance, where a generator is provided with a 150A-class welding machine, the diameter of the rotor core becomes as large as 140 mm and the thickness of the stacked rotor core becomes as thick as about 60 mm. Therefore, after a field winding is wound around the core and further an armature winding is wound around a stator, the total axial length of the welding generator (between both the coil ends) is as long as 120 mm or more. Here, the reason why the size of the welding generator becomes large is that it is necessary to cope with the problem with respect to a rise in temperature caused by the magnetic field winding wound around the rotor. When the size of the welding generator becomes large, since the welding generator is lack of portability, a small-sized and light-weight welding generator usable with the welding machine has been so far needed.
However, when the welding generator is provided with both the magnetic field winding and the armature winding, the temperature within the welding generator tends to rise due to heat generated by both the windings, with the result that the resistance values of the windings increase. Therefore, it has been impossible to reduce the size of the generator, as far as a construction suitable to effectively radiate heat of the two windings cannot be adopted.
For the reason as described above, a basic improvement such that a permanent magnet is used instead of the field winding has been so far required in the field of the portable welding generator.
Here, when considering the field system of a motor having parts or elements usable in common as those of a generator (i.e., the motor and generator are both used as electric rotating machines), permanent-magnetic type motors each having a rotating field system generated by a permanent magnet are widely now on sale, in order to solve a rise in temperature due to heat generated by the field winding. Further, in this case, a rare earth magnet, in particular a samarium-cobalt based magnet is widely used to obtain higher magnetic characteristics
In the samarium-cobalt based magnet, since the magnetic characteristics are sufficiently excellent, there arises no problem, as far as the magnet is used when a steady output is generated as with the case of the motor or the ordinary generator.
In contrast with this, a problem arises in the field of the welding generator. In more detail, in the welding work, since the short-circuit and open-circuit are often repeated, a large reaction magnetic field is inevitably generated. In addition, there exist other problems caused by the vibrations transmitted from the engine (a driving source), the intermittent operation of about 10-min period (which is peculiar to the welding machine), the braking and vibrations due to the repeated large current short-open operation under a non-steady current load, etc. Therefore, a large mechanical strength is required for the permanent magnet.
A practical experiment indicated that the strength of the samarium-cobalt based rare earth magnet is not sufficient when used for the welding generator, with the result that the magnet is easily damaged or broken.
To overcome this problem, a neodymium-iron-boron based rare earth magnetic excellent in mechanical strength has been studied as a rare earth magnet other than the samarium-cobalt rare earth magnet. In other words, a permanent magnetic type welding generator having the neodymium-iron-boron based rare earth magnet arranged on an outer circumferential surface of a rotor as the field magnet has been developed.
In the case where the neodymium-iron-boron based rare earth magnet is used for the welding generator, in order to cool the rotor magnet and to reduce the size and weight of the welding generator, it is preferable to support the engine in cantilever fashion and to introduce cooling air from a surface of the generator housing remote from the engine,
When the welding generator is supported by an engine shaft in cantilever fashion, it is necessary to secure the centering or alignment between the welding generator and the engine by determining a length of a faucet (or spigot) portion of a housing to be relatively long. In this case, in order to entirely fit the stator core to the faucet portion of the housing, it is necessary to determine the contact area between the housing and the stator core to be broad so that the fitting tolerance between both can be loosened. Otherwise, the fastening work and alignment work of both the stator core and the housing become difficult, so that a precise centering between he engine and the generator is difficult to achieve. As a result, the magnetic gap inevitably increases, with the result that the size of the generator tends to be increased.
On the other hand, however, the characteristics between the temperature and the demagnetization rate of the neodymium-iron-boron based rare earth magnet (Pc=1 shown in FIG. 17) are not excellent as compared with those (Pc=2 shown in FIG. 17) of the samarium-cobalt based rare earth magnet. Therefore, in the case where the welding generator temperature rises as high as 100.degree. C. by the heat conducted and radiated by the engine as with the case of the engine-driven welding generator, a problem has been confirmed in that the magnetization characteristics of the permanent magnet cannot be restored. To overcome this problem, therefore, it is necessary to cool the inside of the generator housing sufficiently.